Andy writes:

Look in the tech manual for a spectrum analyzer that covers a few
Khz to 1.3 Ghz and you will see exactly how it's done.

Typically, the LO runs above 1.3 Ghz and all signals are converted
UP to a high IF where the first filter is a wideband SAW. A SAW can
have several Mhz bandwidth way up there.

Then the signal is downconverted to an IF that is compatible with
the SAW bandwidth to achieve image rejection. The IF filter here
is usually something like a crystal or ceramic filter, unless the
requirements
for instantaneous bandwidth forces a better group delay response.

The hardest part is keeping the signal linear ( intermod,
crossmod)
in the presence of multiple simultaneous signals until the SAW and
next IF filters are reached.

Lots of tradeoffs have to be made.

These tradeoffs have already been made in spectrum analyzers, so
if you get a few different block diagrams of different units, you
will
see what specific frequency plans the designers have decided to use.

The frequency plan of the cheapest spectrum analyzer would
probably fit your purpose....

Coming up with a good frequency plan in view of the nonlinearities
of the mixers used and the bandpass characteristics of the filters
one employs is a real challenge sometimes.

Andy in Eureka, Texas